Affirming the consequent

Affirming the consequent, sometimes also called asserting the consequent or the converse error, is a type of logical fallacy where a premise is asserted as true simply because a conclusion implied by the premise is true. This is a fallacy because it assumes that the conclusion could only have been reached in one particular way.

For example:

If an animal is a rat, then it has four legs

My dog Lassie has four legs

Therefore, Lassie is a rat

Formal Construction

A implies B

B

Therefore, A

Further Examples

Creationism example 1

If the universe was created by God, then there would be order and natural laws observed in the universe

We see examples of order and natural laws in the universe

Therefore, the universe was created by God

This argument assumes that a deity is the one and only explanation for order in the universe. And even if we do assume that a deity is the only explanation, it doesn't automatically mean that it must be the Christian deity. The Flying Spaghetti Monster is a good counter-example that satirically exposes the underlying fallacy of this argument.

Creationism example 2

If the Biblical flood story is true, then there was a boat built with the dimensions described in the Bible

We found a boat with these dimensions

Therefore, the Biblical flood story is true.

To see why this is a fallacy, consider the same logic applied to the story of Cinderella and the existence of Cinderella's castle in Disneyland.